1. Field
The present invention relates to a magnetic system for vacuum facility according to the preamble of claim 1. The invention relates in particular to coating facilities with a magnetic system for controlling plasma properties.
2. Description of Related Art
To control the plasma properties in coating facilities, magnetic systems are used among others to generate magnetic fields. The magnetic field lines and magnetizing forces are to be distributed in the coating chamber respectively on the substrate as homogeneously and effectively as possible. An important method for generating magnetic fields is the mounting of coils, especially in a Helmholtz configuration. A practical fitting of coils, e.g. in a Helmholtz configuration, is very difficult to achieve especially in large batch facilities with frontloading, as they would protrude when the doors are opened and stand in the way during loading and unloading.
One way of overcoming this difficulty is to switch from frontloading to bottom and/or top loading. Accordingly, a coating facility and in particular a heating process by means of a plasma are disclosed in U.S. Pat. No. 5,250,779 in the name of Kaufmann et al., wherein the substrates are connected as the anode of the plasma source. The plasma and thus the distribution of the heating output are controlled through a couple of Helmholtz coils. In this process, the loading of the chamber with substrates is carried out as per the concept ‘bottom loader’, i.e. parallel to the coil axes and through the lower coil. As a result, there is no collision with the Helmholtz coils. So-called top loaders, which are loaded from above, can be put into effect in a similar manner.
If the concept of the front loader, however, is to be retained, the aforementioned difficulties will have to be confronted, as are further illustrated in FIG. 1. It shows a perspective view (FIG. 1a) as well as a top view (FIG. 1b) of a coating facility 1 with an open front door 3. It is therefore a front loader. The Helmholtz coils 5, 7 should have nearly the diameter of the coating chamber 9 in order to provide the required homogeneous field in the chamber. The coil 5 is then placed above the actual chamber and the coil 7 is mounted below the chamber. As can be clearly seen in FIG. 1, the coil 7 presents a considerable obstacle for loading and unloading, as it is not possible to approach the frame of the chamber.
A possible solution to the problem is also indicated in FIG. 1, as described in US200601020077 in the name of Esselbach et al. A very small lower coil 11 relative to the ideal Helmholtz configuration is hereby suggested. The coil therefore no longer disrupts the loading process; the achievable distribution of the magnetic field within the chamber is however far from its optimum.
In another attempt at a solution, as a lower coil, a coil is proposed whose windings at certain points are designed as movable strands. This makes it possible for example to fold down a segment of the coil during the loading process. The manufacturing and especially the maintenance of such coils, however, involve substantial efforts and expense. It must notably be ensured that a strong electricity current can flow without any major hindrance through the movable parts. Furthermore, space must also be provided in the loading and unloading area for the part that is to be folded down, so that it is necessary to consider that the coils used for this purpose have a considerable thickness.